This invention relates generally to synchronous detectors and in particular to those using a local oscillator source of carrier injection. In addition, this invention is related to copending application Ser. No. 727,095, filed Sept. 27, 1976, now patent No. 4,072,909, assigned to the assignee of the present invention which is hereby incorporated by reference.
Synchronous detectors typically comprise multipliers or gated type demodulators to recover amplitude modulated information from a received carrier signal. They differ from the more common envelope type amplitude modulation detectors in their requirement of a source of injection signal maintained in proper phase and frequency synchronism with the amplitude modulated carrier. While offering substantial performance advantages over the less complex and cheaper envelope detectors, synchronous detectors are, in some respects, more difficult to utilize effectively. For example, the phase and frequency synchronization requirements for proper synchronous demodulation applicable in most environments impose exacting performance criteria upon the systems used to supply the proper injection signal.
Recently, television demodulation systems incorporating synchronous detection have been developed in increasing number. Basically, a television demodulation system includes a tuner used to frequency convert a selected one of a plurality of available frequency-spaced station carriers to a common intermediate frequency (IF) signal. Precise bandwidth amplifiers amplify the IF signal to a power level sufficient for demodulation. A synchronous detector oscillator supplies an IF carrier-like signal to the demodulator. The most common synchronous detecting system used in television receivers also includes a closed-loop control system operative upon the detector oscillator to provide synchronism between the locally generated "carrier" and the IF signal. In other, less common, television systems, the synchronous detector oscillator is maintained at a constant frequency and a control system is operative on the tuner oscillator causing the frequency conversion system to alter the IF signal to conform to any changes in the detector oscillator signal. Systems using a closed loop operative upon the detector oscillator "follow" the intermediate frequency signal and thus follow tuner adjustment while those operative upon the tuner do not. The selection is largely one of design choice. In either case, the narrow range of frequency and phase error between the IF and carrier injection signals tolerated by such synchronous detectors limits the operation of the system to the performance range of the control loop.
Recently discovered frequency and phase lock loops, such as those set forth in the above-mentioned referent application, provide improved wide range frequency and phase control loop performance, thus opening the way for greater synchronous detector flexibility. However, such systems are characterized by two equal-probability phase synchronization conditions which, when used in an environment such as a television demodulation system, may recover the modulation components at either of two opposite polarities which is of course undesirable.
Accordingly, it is an object of the present invention to provide an improved synchronous demodulation system. It is a more particular object of the present invention to provide an improved synchronous demodulation system suitable for use in a television receiver environment.